He also faces three counts of health and safety failings involving Mrs Coe’s boiler and those of customers in Raydon and Woodbridge.

Sykes, who denies all the charges, had serviced Mrs Coe’s oil heating system for about 40 years.

He carried out the last service at her St Edmunds Road property on October 19, 2012 – just 47 days before she was found collapsed and unconscious by her son.

William Carter, prosecuting, told the court that Sykes’ failure to clean the flue of Mrs Coe’s oil-fired heater caused a blockage, which resulted in “poisonous gas circulating around her home at fatal levels”.

“The defendant’s failure was grossly negligent to the extent, the crown alleges, that his actions amount to the criminal offence of manslaughter,” he said.

The court heard that two of Mrs Coe’s daughters, Sarah Licence and Bridget Coe, had raised the alarm on the evening of December 5.

They became concerned after their mother, who lived alone, failed to answer the phone.

Alerted by Bridget, William Coe arrived at the property at around 11pm.

He noticed the atmosphere in the kitchen seemed “hazy” and entered to find his mother lying unconscious on the floor, the court was told.

She was taken to hospital, where doctors described the level of carbon monoxide as “extremely high” and said nothing could be done. She died on December 6.

Investigations carried out the following day revealed the flue was “completely blocked” with almost 11kg of debris.

Mr Carter, reporting the inspectors’ conclusions, said the defendant’s service had fallen “far short of the expected standards of competency”.

Inspections of other boilers serviced by Sykes also revealed shortcomings, leading to two further health and safety charges.

Mr Carter said an oil boiler belonging to Melvyn Smith in Raydon was “declared immediately unsafe”.

And a Mexico gas boiler, which Sykes had been servicing for Mary Blyton at her bungalow in Woodbridge for the past 15 years, was classified as “not simply unsafe, but as immediately dangerous”.

Sykes, who is not on the gas safe register “should not have had anything to with the gas boiler at all”, the court heard.

During police interview after Mrs Coe’s death, Sykes was said to have claimed he carried out a proper service and cleaned the flue.